This invention relates to a system used for detecting a marker within a predetermined zone. The invention is intended primarily for use in the detection of goods in electronic article surveillance or anti-theft systems, but it may be used for example in article tracking or personnel detection systems.
The invention may be used for any type of anti-theft system wherein a magnetic field is set up in a predetermined zone and a specific disturbance magnetic signal is detected, including so-called `RF` (radio-frequency, generally above 2 MHz) systems, where the disturbance is caused by markers including capacitive/inductive circuits. However, the invention is here illustrated with respect to low-frequency so-called `EM` (electromagnetic, including electromagnetic harmonics) systems, where the interrogating frequency is generally below 100 kHz, and where the disturbance is caused by markers including a magnetic material with a non-linear magnetic response.
The present invention relates to the physical configuration of the field transmission and reception means, and it allows more efficient use of the magnetic field over a larger volume than hitherto possible. The configuration also allows a high degree of control over the regions where markers can be detected throughout a large volume.
Systems of the prior art have generally used antenna assemblies which are designed to project a magnetic transmission or reception field pattern outside the physical extent of the antenna, so that people and articles move past the assembly or through a zone which lies between two separate assemblies. These assemblies may have transmission and reception means more or less co-located, as in EP 0 134 087, U.S. Pat. No. 4,769,631 and EP 0 352 513. Alternatively, two separate assemblies may be used for transmission and reception respectively, as in U.S. Pat. No. 4,994,939 and EP 0 483 786. In all of these systems, the antenna assemblies are generally flat (i.e. thin, less than about 10 cm thick and usually less that 5 cm thick), panel-like or lattice-like in shape, and with the plane of the panel lying parallel to the direction of passage of the people or goods. In the last two examples cited, the transmitter and receiver enclosures, though separate, are generally very similar in shape and appearance, and are shaped and disposed as described above.
One disadvantage of the prior art systems is that the magnetic field which is experienced by the marker is relatively weak compared with that within or close to the actual antenna assemblies, since it lies some distance outside the antenna. In order to achieve a field sufficient for marker detection, high electrical currents must be passed through the transmission antenna, resulting in the need for costly electronics and heat management systems. Alternatively, the interrogation zone must be very narrow and restricted to the area between the two assemblies: generally less than 1 m wide for "EM" systems or 2 m for "RF" systems (which have more sensitive markers). A second problem, then, is that such systems have not been able to cover very wide passages or exits (e.g. over 2-3 m wide).
Furthermore, the antenna assemblies of known electronic article surveillance systems are generally relatively small, usually being no more than 1-1.5 m tall and 0.5-1 m wide. Due to the small size of the assemblies and their positioning relative to each other, the magnetic field generated in the interrogation zone is non-uniform. This means that the locations of the transmission and reception assemblies must be rigidly specified for the system to operate in a reliable manner. This greatly restricts installation flexibility, since it is rarely possible to adapt a system of this type to an existing store layout; rather, the store layout generally has to be completely redesigned so as to accommodate the electronic article surveillance system. In addition, the volume in which markers are detected in such systems generally extends beyond the volume contained between the transmission and reception assemblies. The installation of such a system consequently results in a reduction of the available retail area since it is not possible to display marked goods near the transmission assembly without setting off an alarm. Moreover, an honest customer who has no intention of shop-lifting may accidentally set off the alarm by passing too close to the transmitter assembly, thus leading to embarrassment both for the customer and for the retail store.
Furthermore, recent trends are toward the use of very small markers, which are attractive to the end user but which produce very limited amounts of signal. Because of this, it is very difficult significantly to increase the range of detection of the receiving antennae without swamping the received marker signal in electromagnetic noise generated by the normal environment. Additionally, with an enlarged interrogation zone, it becomes even more important to have some means to limit the overall detection volume in a controllable manner: for example to stop detection in regions which might contain merchandise but which are unavoidably close to the enlarged interrogation zone.
Metal detection systems which utilise a walk-through gate are well-known in connection with airport security systems. These systems generally comprise a single antenna of conducting material through which an alternating current of high magnitude is passed. This sets up an alternating or pulsed magnetic field which induces eddy currents or magnetisation in metal objects which pass through the gate. By their effect on detuning the antenna or on a pulse of magnetic field, the presence of induced eddy currents or magnetisation in nearby metal may be detected. This detection generally takes place by sensing a potential developed across the antenna, or a frequency shift in the antenna resonant frequency; this means that only a single antenna is required for both transmission and detection. These systems, however, allow no discrimination between different ferromagnetic objects, and will give an alarm indication for harmless objects such as belt buckles and key rings as well as for objects such as guns and explosive devices. While such lack of discrimination may be perfectly adequate for use in anti-terrorism applications, where the general public will be prepared to undergo the inconvenience of false alarms, this situation would clearly be unacceptable in electronic article surveillance applications, where a high degree of discrimination is required. Furthermore, in view of the high incidence of false alarms in airport security systems, it is not possible to make the walk-through gate large enough to allow the passage of more than one person at a time, since the alarm would be activated so frequently that it would cause great difficulties for security staff to determine which person was carrying the detected metal article. Because these known walk-through gates only allow the passage of one person at a time, and because of their lack of discrimination, they are completely unsuitable for use in the applications addressed by the present invention.